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Thanks for the answer. Unfortunately it really was all zeros. The gradients were identical. I have just fixed this issue:

The OpenCV library can be used in two ways on an Android smartphone: it can be embedded within an application directly, or installed as an app - "OpenCV Manager", which then provides the libraries to the OpenCV apps dynamically.

My application was using the OpenCV Manager app to get the libraries. Unfortunately, it appears that OpenCV Manager for Android is bugged, because when I embedded the OpenCV library directly within my app, the Sobel calls started working properly.

So I recommend everyone to link their applications to OpenCV statically.